Methane emissions from rice fields, such as this one in India, have risen in recent years.Arun Sankar/AFP/Getty Images

The Greenhouse Gas Riddle

What is Causing the Recent Rise in Methane Emissions?

The cause of the rapid increase in methane emissions since 2007 has puzzled scientists. But new research finds some surprising culprits in the methane surge and shows that fossil-fuel sources have played a much larger role over time than previously estimated.

The stomachs of cattle, fermentation in rice fields, fracking for natural gas, coal mines, festering bogs, burning forests — they all produce methane, the second most important greenhouse gas, after carbon dioxide. But how much? And how can we best cut these emissions? And is fracking frying the planet, or are bovine emissions more to blame?

Until now, the world has not had a definitive answer to these questions. But in recent months, researchers believe they have finally begun to crack the problem — and the results are surprising.

The amount of methane in the atmosphere has more than doubled in the past 250 years. It has been responsible for about a fifth of global warming. But it has a confusing recent history. The steady rise of emissions stopped in the 1990s. Emissions were stable for almost a decade until 2007, but then abruptly resumed their rise.

What has been going on? Fracking of natural gas in the U.S. and elsewhere has frequently been blamed for the resumed rise in emissions. But new studies are raising serious questions about that.

Researchers are now saying say that, globally at least, the increase in recent years is due to the activities of microbes in wetlands, rice paddies, and the guts of ruminants. “Despite the large increase in natural gas production, there has not been an upward trend in industrial emissions,” says Stefan Schwietzke, of the U.S. National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colo., who is the lead author of one of the new studies.

“Since 2007, atmospheric methane has been rising rapidly,” notes Euan Nisbet of Royal Holloway, University of London. But he says his research shows that the increase is dominated by microbial emissions, particularly from the tropics.

Yet that hardly exonerates gas fracking. It turns out that, all along, natural gas and other fossil fuels have been a bigger source of methane emissions than the industry has declared in submissions to governments and the UN. The companies may not have been deliberately lying; but the new studies prove that they were certainly and comprehensively wrong.

The new data come from combining two approaches to unraveling the methane riddle. First is a new database of isotopic analyses of methane in the air. This analysis can distinguish between methane coming from the three main categories of sources: the output of microbes living in anoxic environments such as wetlands, landfills, and the stomachs and butts of ruminants; fossil methane in gas, coal, and other underground fuel reserves that is released as those reserves are exploited; and the burning of vegetation like forests, bush, and crop residues.

By combining research approaches, a previously fuzzy image of methane sources is becoming dramatically clearer.

Methane molecules from each of these sources have a characteristic ratio of two carbon isotopes, carbon-12 and carbon-13. Microbes fractionate the isotopes, increasing the proportion of carbon-12 compared to fossil methane. Yet methane from burning trees or crop residues has a higher proportion of carbon-13, because photosynthesis favors that isotope.

Second, satellite observations of methane concentrations in the air are allowing researchers to accurately identify the regions of the world from which increases in emissions are occurring. As the two approaches are combined, a previously fuzzy image of methane sources is becoming dramatically clearer.

The most detailed isotopic analysis was published earlier this month by NOAA’s Schwietzke in the journal Nature. He developed a global database of isotopic measurements taken all over the world in the past three decades and analyzed the trends. Two surprising discoveries emerge from Schwietzke’s study. First, the recent methane surge into the atmosphere is due not to the rising fossil-fuel emissions, but rather to an unexpected surge in microbial sources. Second, fossil-fuel sources are — and have been for at least some decades — almost twice as big as previous estimates, whereas microbial sources are about a quarter less.

Microbial methane still accounts for the majority of emissions, totalling almost 400 million tons a year, but fossil-fuel emissions are much more significant than previously thought, at about 200 million tons.

Methane lasts in the atmosphere only for about 12 years, much less than CO2; but while there, it packs a punch. Measured over a century, a molecule of methane warms the planet roughly 30 times more than CO2.

The findings from the latest methane studies have caught scientists by surprise. But they are backed up by other recent research.

In a separate paper published less than two weeks before Schwietzke’s, Nisbet combined a smaller set of isotopic data with a detailed analysis of the geographical sources of recent methane emissions. And he reached a similar conclusion. “Our results go against conventional thinking, but the analysis clearly points to increased emissions from microbial sources,” he said.

Do these trends matter?

For more than two centuries, rising methane emissions have resulted in steady increases in the concentration of the gas in the atmosphere. In fact, while CO2 concentrations have so far risen by only about 40 percent since pre-industrial times, methane levels have more than doubled, rising from 700 parts per billion to almost 1,800 ppb.

But the rise slowed in the 1990s, then showed a brief surge in 1997 when the large El Niño that year triggered a surge in emissions from burning forests, and then came to a complete halt from 1999 to 2006. Before scientists had figured out why the rise ended, it resumed. Now, finally, they are catching up with events.

A chart showing sources of global methane emissions.NOAA

A study published earlier this year by Hinrich Schaefer, of New Zealand’s National Institute of Water and Atmospheric Research, using isotopic data concluded that the cause of the decade-long hiatus in rising methane emissions was the fallout from the breakup of the Soviet Union in 1991.

Soviet gas production had long been suspected of being extremely leaky, because of both poor production technology and heavy leaks from its long pipelines carrying huge quantities of natural gas from Siberian fields to Europe. The collapse of the Soviet Union both reduced production and brought in Western technology that helped cut the leaks. Now it is becoming clear that the resumption in the rise in emissions after 2007 is a result of a surge in activity by methane-producing microbes. And geographical data from Nisbet and others show that the main zones of new emissions are the tropics. So what has happened to get the microbes there so excited?

Some researchers are blaming farmers. Schaefer argues that most of the post-2006 increase in methane emissions emanated from China, India, and Southeast Asia. In these growing and densely populated countries, which have only small areas of natural ecosystems, it seems that increased production from flooded rice fields and a growing number of livestock have been the likely culprits.

But Nisbet thinks the weather, acting to enhance microbial activity in both natural wetlands and flooded rice paddies, is more likely to blame. He says since 2007, rising temperatures and changes in rainfall have created ideal conditions for microbial methane production. For example, many wetlands have been enlarged by heavy rains. He says the tropics have seen a long run of wet years, topped off with an exceptionally warm 2014, which coincided with the biggest surge in methane emissions so far. It may not all be occurring in the tropics, he concedes. There was a local spike in Arctic emissions in 2007 during exceptionally warm conditions that summer, during which methane may have been released from melting permafrost. But his analysis suggests that spike was not repeated in subsequent years.

In any case, whether the source is the Arctic or the tropics, Nisbet warns that there is a real danger that climate change is starting to accelerate the processes that release methane into the atmosphere, potentially triggering a troubling positive feedback in which further warming could produce more methane and yet more warming. He calls the warmer and wetter conditions of the past decade, and their apparent impact on methane production across the tropics, “a troubling harbinger of more severe climate change.”

Where do we go from here? Now that the world has a strategy — agreed to in Paris last December — for combating CO2 emissions, it badly needs a similar plan for climate enemy No. 2: methane. The Paris agreement to keep warming “well below two degrees C” will be unachievable without it.

So if microbial emissions are behind the recent rise, what can be done to reduce them? Agriculture may be able to help. Research suggests that a reduction in the amount of time that paddies are flooded can reduce methane emissions without sacrificing productivity, and animal breeders are working on producing cattle with guts that produce less methane.

A fifth of natural gas wells accounted for more than three-quarters of the venting of methane to the atmosphere.

But the fossil fuel industry is likely to face even more pressure to act. The industry will of course point out that the new evidence shows that natural gas producers in particular have been reducing their emissions while increasing production. Schwietzke reckons that natural gas production was probably responsible for about 8 percent of total methane emissions per unit of production in 1985, but that this figure, including fracking, could now be as little as 2 percent.

The discovery that the industry has been systematically underestimating its contribution to climate change for decades — and that national methane inventories submitted to UN climate negotiators are plain wrong — is bound to increase pressure for action. And with good reason.

The extent of how much could still be achieved was underlined two years ago by David Allen at the University of Texas at Austin, who gained unusual access to U.S. fracking wells to assess where their leaks were coming from. He found that “a small subset of natural gas wells are responsible for the majority of methane emissions.” A fifth of wells accounted for more than three-quarters of the venting of methane to the atmosphere.
“Much more can be done to reduce fossil fuel emissions,” says Nisbet. “This is one of the quickest and most cost-effective ways of cutting greenhouse gas warming.”

Correction, October 28, 2016:Earlier versions of this article incorrectly described Stefan Schwietzke’s estimate of the change in total methane emissions since 1985. Schwietzke said that natural gas production was probably responsible for about 8 percent of total methane emissions per unit of production in 1985, but that this figure could now be as low as 2 percent.

Fred Pearce is a freelance author and journalist based in the U.K. He is a contributing writer for Yale Environment 360 and is the author of numerous books, including "The Land Grabbers, Earth Then and Now: Potent Visual Evidence of Our Changing World," and "The Climate Files: The Battle for the Truth About Global Warming." More about Fred Pearce →